SAT Math Multiple Choice Practice Question 128: Answer and Explanation

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Question: 128

18. The expression (x - 5)2 is equivalent to

A. x2 - 10x + 25
B. x2 + 25
C. x2 - 10
D. x2 - 5x + 25
E. x2 - 25

Correct Answer: A

Explanation:

A. Ah, yes, an old trap from my algebra classes. To solve this problem, you can use a formula I explain in Chapter 14: (a + b)2 = (a + b)(a + b) = a2 + 2ab + b2. I'd memorize that formula, if I were you. But, if your memory freezes, just use the FOIL method by writing (x - 5)(x - 5). Now follow the FOIL steps. Multiply the first terms (x · x = x2). Now multiply the outer terms (x · -5 = -5x). Next, multiply the inner terms (-5 · x = -5x). Now multiply the last terms (-5 × -5 = 25). Combine the results, and you end up with Choice (A).

Check out Chapter 14 for more details about factoring and using the FOIL method.

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